In addition to actual information, telecommunications systems must transmit control information to guarantee successful flow of information between the sender and the receiver. The required control information includes, for example, channel addressing, which indicates the transmission channel to the receiver. Additionally, the parties of the connection must agree upon connection set-up before data transmission and connection set-down after the transmission. In mobile communications systems, for example, the base station system must also locate the mobile station before connection set-up.
Channels can be defined as logical and physical channels. The term “logical channel” refers to a channel whose use is in some way determined. For example, traffic channels are used for transmitting user information and signalling channels for transmitting control information required for connection management. Signalling channels can be further divided into connection-specific channels and shared channels. In the case of a connection-specific channel, the channel itself determines the receiver of the message. In the case of a shared channel used by several connections, the receiver's identity is indicated by adding the receiver's identifier to the message. As a result, messages are slightly longer in shared channels than in connection-specific channels even though the actual information content is the same.
Connection-specific and shared signalling channels can be further divided, when necessary, into subcategories. In a mobile communications system, for example, shared signalling channels can be divided as follows: the Broadcast Control Channel (BCCH) intended for transmitting network managing information for all mobile stations, the Paging Channel (PCH) which is used to send paging messages to specific mobile stations, and the Access Grant Channel (AGCH) which is used in call set-up.
The term “physical channel” refers to a specified section of the transmission band. In an FDMA/TDMA (FDMA=Frequency Division Multiple Access, TDMA=Time Division Multiple Access) system, for example, the physical channel consists of a specific frequency and time frame interval. Logical channels are mapped to physical channels so that a specific physical channel always provides for a specific logical channel. The information about the mapping of logical channels to physical channels must naturally be known both by the sender and the receiver, and, therefore, this information must be transmitted, during connection set-up phase, via predefined signalling channels, such as AGCH.
In known systems, there is a specific logical channel for each message that transmits control information. An example of such a system is shown in FIG. 1. The figure displays the transmission of four different types of control information in a mobile communications system from the Base Station Subsystem (BSS) to the Mobile Station (MS). The message that conveys the Power Control (PWC) command for the Mobile Station, is sent via the Slow Associated Control Channel (SACCH). Correspondingly, all messages which relate to handover are sent via the Fast Associated Control Channel (FACCH). Messages which are related to paging of Mobile Stations are sent via the Paging Channel (PCH), and messages which are related to connection set-up before the allocation of a connection-specific channel are sent via the Access Grant Channel (AGCH).
In this example, intervals 0 through 22, defined by the physical channel are in connection-specific use of the connection under study, and intervals 23 and 24 are used by signalling channels shared by several connections, and interval 25, which defines the end of the frame, is empty. The SACCH is set, in accordance with system specifications, in interval 12 of a frame which consists of 26 consecutive intervals. In the figure, interval 12 is indicated by the symbol S which also signifies the logical channel. Thus, the receiver always knows that the information received through interval 12 belongs to the SACCH, and, on the basis of that knowledge, can interpret the message correctly. Correspondingly, all messages received through interval 23 belong to the logical Paging Channel (PCH), and all messages received through interval 24 belong to the logical Access Grant Channel (AGCH), and, on the basis of this knowledge, the receiver can interpret them correctly.
Contrary to other logical channels, the mapping of the Fast Associated Control Channel (FACCH) to a physical channel has not been set by signalling or system specifications. Instead, it can use any interval T allocated to the traffic channel. In this case, the logical channel used in the interval must be indicated in the actual information sent through the interval. Known methods of separating the FACCH from the Traffic Channel are shown in Figure.
FIG. 2 shows a burst used in the call traffic across the radio interface between a Mobile Station and a Base Station Subsystem in a GSM system. The effective part of the burst consists of the first and second half-burst, their two signalling bits (“stealing bits”), and the instruction sequence used to estimate the channel characteristics. In this burst type, the first half-burst belongs to the signalling traffic of the logical FACCH channel if the first signalling bit is 1, and otherwise to the traffic of the Traffic Channel (TCH). Correspondingly, the second half-burst belongs to the signalling traffic of the logical FACCH channel if the second signalling bit is 1, and otherwise to the traffic of the Traffic Channel (TCH). Thus, it is possible to use the traffic channel burst for signalling either partially or entirely.
Different logical channels have different characteristics. Because of its limited physical channel capacity, the Slow Associated Control Channel (SACCH) is slow, and, therefore, it can only be used to transfer relatively small and delay-tolerant information streams. Another problem for this logical channel is that the channel reserves transmission resources available to the system even when it does not have any messages to carry. In a GSM system, for example, SACCH is used for downlink control of power and timing advance (from the Base Station Subsystem to the Mobile Station), and for uplink reporting of received signal measurements made by the Mobile Station (from the Mobile Station to the Base Station Subsystem).
The Fast Associated Control Channel (FACCH) is considerably faster than the Slow Associated Control Channel SACCH, because it can use the bandwidth allocated to traffic channels. On the other hand, the bandwidth adopted by FACCH from the Traffic Channel is no longer available to the Traffic Channel, resulting in the deterioration of the Quality of Service (QoS) of the Traffic Channel. In a GSM system, for example, FACCH is used to send information, such as messages related to call setup, authentication and handover.
The capacity of shared channels is limited and used by several Mobile Stations. In some cases, this can increase the transmission delay of messages sent via a shared channel. This is the reason why shared channels are used, in an existing GSM system, for example, only for sending messages before connection set-up between the transmitter and receiver. Such messages include paging messages and connection set-up messages.
The problem with solutions that are in accordance with the prior art is the rigidity of the signalling method described above. When large numbers of messages are transmitted via the Fast Associated Control Channel (FACCH), which uses the capacity of the Traffic Channel, the quality of the connection using the Traffic channel deteriorates. Other connection-specific channels reserve transmission capacity available to the system. Choosing the level of this capacity is a compromise between the signalling speed and the bandwidth allocated to channels. This results in slow signalling when a relatively large number of messages is generated. Because of slow signalling, the system control capacity deteriorates which, in turn, results in the non-optimal use of other resources. Correspondingly, when only a few signalling messages are sent, a separate channel allocated to signalling is a waste of system resources. Additionally, slow signalling channel allocation is typically connected to traffic channel allocation. As a result, the use of a slow signalling channel may, in some cases, cause a need to maintain the Traffic Channel even though the Traffic Channel is no longer needed for the transmission of user data.
If the shared signalling channels are to transmit messages at the rate required by the system, they must be allocated a fixed share of the transmission capacity available to the system. This capacity cannot be allocated to traffic channels. Because the transmission of messages via a shared channel is statistical in nature and varies considerably over time, some of the transmission resources allocated to the shared channel are left unused.
Thus, the problems in systems which are in accordance with the prior art include slowness of signalling, deterioration of connection quality, and non-optimal use of the transmission band.
The purpose of this invention is to remove or at least alleviate the problems caused by the above-mentioned solutions that are in accordance with the prior art. This goal can be attained by using the method and equipment described in an independent patent claim.